I always suggest a Silithid at low resilience, and any Spirit Beast once you're geared.
SB throws out a lot of heals & has the best solo-play damage buff in the game, and it does tons of damage itself as it's Ferocity.
Silithid has a ranged root + stamina buff, + Roar of Sacrifice for PvP, all of which are excellent when you're low on resi and need to play more defensively.
Chimaeras have a nice ranged snare, and if it's over you when you FD enemies may have trouble retargeting straightaway, but I found the Silithid's root more useful (and the stamina buff is nice for when it's lacking).
As to WHICH spirit beast--they're all pretty similar. If you want to be easily able to keep track of it in a fight, Skoll is best for color or Loque for sound if you play with sound. If you want your pet to not get focused, use Ghostcrawler--it's tiny, and translucent, and a lot of people don't seem to notice it eating their butts off. He's also the most likely to be spawned when you go hunting for a spirit beast, since he's unkillable, only tameable, and very out-of-the-way.
As mentioned, Devilsaurs do put out a free MS, + heavy damage, so if you're running with a healer + already have the spirit beast's buff, a Devilsaur is great to have.
Bring a Core Hound too. You can pop Bloodlust at the start of a fight in bgs, and then swap pet and go in for the nuke, or just use the Core Hound (it has a ranged casting speed debuff).